Drivers seat slight "knock"

[Chri5]

For the past few months my seat seems to be making a little knocking noise when my weight shifts ever so slightly, say when I go over small bumps in the road.
Its the right hand side of it, that seems to do it.

If I move the seat on the slider sometimes it will go away for a while, sometimes days and then return! Its doing my swede in!!

Any ideas what it could be? Is it common? :S